1. Field of the Invention
This invention relates to analog-to-digital converters (ADCs), and more particularly to ADCs which employ a sigma-delta modulator in the conversion circuit.
2. Description of the Related Art
There is a need for a single-wire system that can be used for the remote sensing of phenomena such as temperature, with only a small integrated circuit (IC) chip. The chip should be able to convert an analog representation of the sensed phenomenon to a digital signal, for transmission to a remote location over the single line. The conventional method for deriving a digital representation of a temperature signal involves the use of a discrete temperature-to-voltage transducer, followed by a voltage-to-frequency converter (VFC), with the frequency output interpreted by a microcontroller or counter circuit that is at a remote location from the temperature sensor. This type of system is described in Transducer Interfacing Handbook--A Guide to Analog Signal Conditioning, ed. by D. Sheingold, Analog Devices, Inc., 1981, pages 125-126. The VFC typically includes a voltage reference and an off-chip resistor/capacitor network to achieve a time reference that is insensitive to temperature changes. Typical capacitor sizes are in the tens of nano-farads, which is too large to implement on a practical IC chip.
Sigma-delta modulators can also be used for an analog-digital conversion. Such modulators produce an output pulse stream in which the pulse density is proportional to the input signal, and are described in general in Electronics Engineers' Handbook, Ed. by Fink et al., McGraw-Hill Book Company, 3d Ed. 1989, pages 14.39-14.41. However, sigma-delta modulators require a clock, and it is impractical to design an on-chip clock with a small enough frequency variation over temperature to be used as a time reference for temperature sensing. If the clock is to be integrated on the same small chip with the remainder of the modulator, two output lines must be provided: one for the modulator output, and one for the clock signal for use in synchronizing the decoding of the modulator output at the receiver. But, as stated above, a single-line output would be highly desirable.
A modified sigma-delta modulator circuit is described in Sarhang-Nejad, "Realization of a High-Resolution Multibit .SIGMA..DELTA. Analog-Digital Converter", Ph.D. Dissertation, University of California at Los Angeles, 1991. This circuit adds two counters to a conventional sigma-delta modulator. The first counter counts up the output pulses from the modulator, while the second counter counts up to full scale and stops the first counter after a preset time period. The count which the first counter has reached at the time it is stopped is taken as a representation of the analog input to the modulator, and is transmitted as a digital word. The second counter is used to both operate the sigma-delta modulator, and to freeze the first counter's count at the end of the preset time period. While this circuit operates with a one-line output, the area occupied by the two counters is too large to integrate the entire circuit on a single practical chip.